There are some images so powerful they can never be forgotten. That was my experience of a famous photograph of the late Princess Diana of Wales. At first glance, the captured scene looks mundane: smiling warmly, the princess is shaking the hand of an unidentified man. But it’s the photograph’s story that makes it remarkable.

On April 19, 1987, when Princess Diana visited London Middlesex Hospital, the United Kingdom was engulfed in a wave of panic as it confronted the AIDS epidemic. Not knowing how the disease—which often killed with terrifying speed—was spread, the public at times treated AIDS victims like…